The invention is based on an emergency hydraulic control.
From German Patent Disclosure DE 196 09 785, which had not been published before the priority date of the present application, a similar emergency hydraulic control for an electronically controlled continuously variable transmission (CVT) is known. The CVT described there, which is preferably used in automobiles, has a controller for emergency operation that in the event of failure of the electrical triggering of the normal driving mode causes a switch to a shifting position with a high gear ratio, in order to create more-favorable restarting and startup conditions. In this controller, among other things, the ratio between the primary and secondary clamping force between the traction means of the pairs of bevel gears is kept constant via an intermediate gear ratio range. In the intermediate gear ratio range, the gear ratio varies as a function of the torque demand of the power takeoff shaft. At a high torque demand, a high gear ratio is selected, which allows starting up from a stop, while at a low torque demand, that is, when an overrunning moment occurs, a low gear ratio is set. The means required to accomplish this will be discussed in the description of FIGS. 1 and 2.
The emergency hydraulic control according to the invention is needed to enable using a continuously variable loop transmission over the entire gear ratio range without reinforcement from an electrohydraulic triggering that is active during the normal driving mode. With the aid of the emergency hydraulic control, on vehicle starting a startup gear ratio in the low range is to be set, for instance to enable starting up on an uphill slope or when leaving an underground garage. The emergency controller should also set a gear ratio in the xe2x80x9coverdrivexe2x80x9d range at high vehicle speeds and low engine torque. What is important in this respect is that during overrunning, for instance when the vehicle is being braked, the gear ratio not drop all the way into the xe2x80x9coverdrivexe2x80x9d range and that immediately after a new rapid rpm increase of the vehicle motor dictated by startup or acceleration procedures, it adjust [?] itself in the direction of xe2x80x9clowxe2x80x9d.
In the emergency hydraulic control of the invention, the clamping force ratio is varied as a function of the volumetric pumping flow supplied or of the engine rpm. To that end, at least one throttle valve is disposed in an open hydraulic circuit for supplying the respective piston chambers of the secondary and primary axial adjusters. The volumetric-flow-dependent pressure change established at the throttle valvexe2x80x94reinforced by further valvesxe2x80x94varies the contact pressure in the corresponding pair of bevel gears. To regulate the primary and secondary pressure, a respective primary and secondary valve is used. Each valve is triggered in turn via a respective electromagnetically actuated pilot control valve in the pilot control valve that, opens during emergency operation, and the pilot control valves are connected among others to the outlet of the secondary valve via orifice valves. The pressure fluid pressure in the primary piston chamber is additionally limited by a of by. The outlet of this pressure limiting valve is connected, via a control line, to the control line located between the primary pilot control valve and the primary valve. At the same time, an orifice valve is disposed between the discharge point of the control line, disposed at the outlet of the pressure limiting valve, into the control line arriving from the pilot control valve and the branching point of this control line from the lead line to the primary-side pilot control valve.
Among others in the overrunning mode, that is, when the gear ratio is moving in the direction of xe2x80x9coverdrivexe2x80x9d, upon a brief increase in engine speed in the emergency mode, this hydraulic circuit reacts with a primary pressure rise, at which the limiting pressure of the primary pressure limiting valve is exceeded.
The pressure fluid that as a result briefly backs up in the control line is between the orifice valve and the control input of the primary valve, moves the primary valve into a return position, in which the primary piston chamber is relieved to the tank. This increases the gear ratio of the loop transmission, thus enabling re-acceleration of the vehicle.
Further details of the invention will become apparent from the dependent claims, which are not recited at this point, and the drawing descriptions.